The present invention relates to a tubular ceramic body for gas passages of a cylinder head in an internal combustion engine.
Tubular ceramic bodies of the above-mentioned general type are known in the art. One of such bodies is disclosed in the German Patent No. 3,346,394. The ceramic body disclosed in this reference has a bifurcated shape. For supporting both tubular portions of the body, a connection is provided, which must counteract the pressures occurring during casting of the ceramic body in the cylinder head with a sufficient resistance. The connection must be composed of zirconium oxide applied by plasma spraying, while the ceramic tube must be composed of tialite (aluminum titanate).
The above-described ceramic body possesses the disadvantage in that for its manufacture it is necessary to form the ceramic pipe and connect different materials having significantly different thermal expansion coefficients and moduli of elasticity. Thereby during application of temperature, first of all during cast and also during the operation, different thermally induced stresses are produced in the ceramic body which includes elements composed of different materials. Therefore there is a danger that cracks can occur in the ceramic body, especially at the contact surfaces of both materials. Further disadvantages are stress peaks which can be expected in the ceramics and also in the casting material. They occur during cooling of the melt in connection with the significantly different deformation properties of different materials (modulus of elasticity). The use of a single material for bringing the connection on the finally sintered tubular body during a second heating step would lead only to a limited result. The reason is that because of different shrinkage of the sintered pipe and a connection which has to be sintered, cracks can occur in the body. A further disadvantage is that only a low connecting strength is produced between the connection and the tubular body itself. In the connection applied by plasma spraying, even in the event of the use of a same raw material, a different joint structure is produced between the ceramic body which is made by slip casting and subsequent sintering, and the connection formed by plasma spraying. Thereby the above specified disadvantages are caused. It has also been shown that especially inner sides of the tubular portions, or the positions at which a support must be provided by means of the connection, are especially endangered during casting of the ceramic body into metal alloys.